All Scheduled Events

June 19, 2013 Wednesday 2:00 PM 
Meyer 5th Fl. CCPP Lounge
Other CCPP (ccpp)

Special HEP Seminar
Sergio Ferrara
CERN

Duality, Black Holes and Groups of Type E7

June 20, 2013 Thursday 11:00 AM 
Meyer 424B
Hard Condensed Matter Seminars (hcmp)

Stephane Andrieu
Jean Lamour Institute

Origin of Perpendicular Magnetic Anisotropy in Co/Ni (111) Epitaxial Layers

Manipulating the magnetization of thin layers by using other routes than applied magnetic field is of particular importance for spintronics applications. One way is to use the Spin Transfer Torque (STT) phenomenon in order to switch the magnetization in nanopillars using a current. Perpendicular Magnetic Anisotropy (PMA) is particularly interesting to obtain since the STT critical currents are strongly decreased compared to devices with in-plane magnetization. We observed that magnetization in Co/Ni (111) superlattices grown by Molecular Beam Epitaxy is perpendicular for a Co thickness range between 0.2 and 3.5 ML [1]. In this talk, we give a review on the structural and magnetic properties of these Co/Ni(111) single-crystalline layers in order to try to understand in details the origin of this PMA [2]. Whereas this perpendicular anisotropy is explained by taking into account a strong interfacial anisotropy, the possibility of a magnetoelastic contribution is analysed here. By using both Transmission Electron Microscopy and X-Ray diffraction, the magnetoelastic contribution in the total magnetic energy is thus estimated and shown to be much smaller than the interface anisotropy. On the other hand, the magnetic properties were also analysed by using synchrotron radiation facilities. Spin polarized photoemission experiments were performed on the CASSIOPEE beamline at SOLEIL synchrotron [3]. The spin polarization at the Fermi level is found to be near 80% for 1ML Co, twice the value measured on a reference Co(111) bulk sample. Finally, recent X-Ray Magnetic Circular Dichroism experiments performed at SOLEIL synchrotron allow us extracting the true spin moment and the in-plane to out-of-plane orbital magnetic moment variation Δm┴-Δm// responsible for the PMA [3]. Surprisingly, clear orbital moment anisotropy is observed for Co but not for Ni, whereas the PMA was always attributed to Ni in the literature. Moreover, the total magnetic moment of Co is found to be strongly enhanced (2.5μB at 20K) compared to bulk (1.7μB) whereas a small increase is expected using DFT calculations (5%). The origin of such discrepancies will be discussed.
[1] - Strong perpendicular magnetic anisotropy in Ni/Co(111) single crystal superlattices S. Girod et al, Appl. Phys. Lett., 94, 262504, (2009)
[2] - Co/Ni(111) superlattices studied by microscopy, X-ray absorption and ab-initio calculations M. Gottwald et al, Phys. Rev. B 86, 014425 (2012)
[3] – to be published

June 21, 2013 Friday 2:00 PM 
Meyer 6th Floor Conference Room
Hard Condensed Matter Seminars (hcmp)

Cyrus Hirjibehedin
London Centre for Nanotechnology

Exploring the Atomic and Molecular Foundations of Nanospintronics

The drive to continue Moore’s Law by shrinking electrical components down to the ultimate limit has led to a great deal of interest in atomic and molecular-scale electronics, in which individual atoms and molecules can be used as circuit elements. More recent proposals also seek to exploit the magnetic properties of these nanoscale objects in new applications in information technology and spintronics. In typical device geometries, the magnetic element is coupled to electrical leads, and these interactions can strongly affect the properties of the quantum system.
Using scanning tunneling microscopy and spectroscopy, we study the effects of interactions between individual magnetic atoms and molecules that are separated from an underlying metallic surface by a thin-insulating layer of copper nitride (Cu2N). For Co atoms on large Cu2N islands, we find that exchange coupling of the spin to the metallic bath can result in Kondo screening as well as dramatically shifting the energy levels of the spin and modifying its effective magnetic anisotropy, the property that determines the stability of its spin orientation. By controlling the exchange coupling, we can tune both the Kondo screening of the systems as well as the anisotropy energy over a broad range of values. Furthermore, this system constitutes one of the few cases in which an open quantum system’s energy levels, rather than just its excited-state lifetimes, can be controllably and observably renormalized. We also study the electronic transport through individual metal-doped phthalocyanine molecules on Cu2N and explore how it can be influenced through the magnetic properties of the molecules. These results have profound implications for controlling the properties of the smallest possible nanoscale spintronic devices.

June 24, 2013 Monday 11:00 AM 
Meyer 611
Hard Condensed Matter Seminars (hcmp)

Stephane Andrieu
Jean Lamour Institute

MBE grown MgO-based Magnetic Tunnel Junctions: Physics and Interests

In 2001, huge Tunnel MagnetoResisance (TMR) was theoretically predicted in the single-crystalline Fe/MgO/Fe(001) system. Such high TMR values were experimentally confirmed in Fe/MgO or FeCo/MgO(001) based Magnetic Tunnel Junction (MTJ) by several groups in the world since 2001. The basis of spin polarized transport in fully epitaxial MgO-based MTJ is explained by considering the particular electronic structure along the BCC(001) structure and the symmetry filtering of the MgO barrier [1]. However, the TMR observed in real systems is not as large as the TMR predicted theoretically. Many assumptions were thus proposed to account for lower values in experiments (impurities at the interfaces, vacancies in the barrier, surface states). The strategy of our group was to look systematically on the special properties of the MBE-grown MTJs and to answer to many questions: Is the interface as good as in the calculations [2-4]? Are there any impurities at the interface? What are the consequences of the interface doping with C or O [5]? What about defect in the MgO barrier [6]? Is there some interface electronic state [7]? What is its impact on the tunnelling process? To highlight these different points, we first studied the effect of Fe/MgO interface doping with O, showing that it does not lead to detrimental effect on the TMR. Second, the use of FeVx electrodes instead of Fe (increasing x decreases the misfit with MgO) allows us to clearly show that the dislocation density in the MgO barrier strongly affects the TMR. Moreover, spin and symmetry resolved photoemission was used to show that a 2D state fully down polarized actually exists at the FexCo1-x/MgO interface and can be detrimental to the TMR for high Co content. Finally, MgO-MTJs with perpendicular magnetic anisotropy are now well-controlled [8] and I will show the effect of an electric field on this anisotropy using an MTJ as a detector. [1] W. H. Butler & al, PRB 63, 054416 (2001); J. Mathon et al, PRB 63, 220403 (2001)
[2] Polarization of (001)Fe covered by MgO analysed by spin resolved X-ray photoemission spectroscopy M. Sicot et al, Phys. Rev. B, 68, p.184406, (2003)
[3] Static and dynamic aspects of spin tunneling in crystalline MTJs, C. Tiusan et al, J. Phys.: Condens. Matter 18 , 941-956 (2006)
[4] Atomic scale study of interfaces in epitaxial Fe/MgO/Fe MTJs, V. Serin et al, Phys. Rev. B 79, p.144413, (2009)
[5] Consequences of interfacial Fe-O bonding & disorder in epitaxial Fe/MgO/Fe(001) MTJs, F. Bonell et al, Phys. Rev. B, 79, 224405, (2009)
[6] Influence of misfit dislocations on the magnetoresistance of MgO-based epitaxial MTJs F. Bonell et al, Phys. Rev. B, 82, 092405 (2010)
[7] Probing deeper into the spin polarized electron tunneling in bccFeCo/MgO/FeCo(001)MTJs F. Bonell et al, Phys. Rev. Lett., 108, 176602 (2012)
[8] Quantifying perpendicular magnetic anisotropy at the Fe-MgO(001) interface C.-H. Lambert et al, Appl. Phys. Lett. 102, 122410 (2013)

June 26, 2013 Wednesday 2:00 PM 
Meyer 5th Fl. CCPP Lounge
Other CCPP (ccpp)

Special HEP Seminar
Antonio Grassi
University of Piemonte Orientale

Integral Forms, Entropy Current and Thermodynamics

I present an overview on integral forms needed for formulating a sensible integration theory on supermanifolds. I discuss the applications of integral forms to thermodynamics and a novel formula for the entropy current based on integral forms in supersymmetric field theory and supergravity.

September 11, 2013 Wednesday 2:00 PM 
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)

Saso Grozdanov
Oxford

TBA

October 9, 2013 Wednesday 2:00 PM 
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)

Alexander Vilenkin
Tufts

TBA

October 16, 2013 Wednesday 2:00 PM 
Meyer 611
Hard Condensed Matter Seminars (hcmp)

Louis Bouchard
UCLA

Nanoscale Studies of Electronic and Magnetic Properties in Topological Materials

In recent years the emergence of gapless topologically protected edge states in the solid state has led to searches for new phases of condensed matter in new and existing materials. For example, some thermoelectrics and Kondo insulators have been shown to be topological insulators. The protected edge states in topological insulators are due to the combination of spin-orbit coupling and time-reversal invariance. Examples of exotic phenomena include the quantum anomalous Hall effect, fractional quantum anomalous Hall effect, topological superconductor, fractional time-reversal invariance, topological Kondo insulator, topological crystalline insulator and the topological magneto-electric effect. However, the interesting properties of topological materials are found at edges and interfaces, making them challenging to study from the experimental standpoint. In this talk, we will review recent advances in experimental techniques to study the electronic and magnetic properties of such topological materials. Among the novel techniques, we shall discuss radioactive ion beam spectroscopy, electrically-detected electron spin resonance and nuclear magnetic resonance.
Our group has been carrying out experiments at TRIUMF using low energy spin-polarized muon and lithium ion beams to resolve properties as function of depth, with nanoscale resolution. Such studies reveal substantial modulations of the material properties at these length scales, which could have implications in the design of devices and in the search for new phases. Studies of material defects in the bulk will also be discussed. Results from these ongoing experiments as well as other experiments will be discussed. Ultimately, the development of new experimental methods is expected to lead to not only insight for improving material properties but may also enable the development of composite materials with optimized properties.

November 22, 2013 Friday 2:00 PM 
Meyer 5th Fl. CCPP Lounge
Astrophysics and Relativity Seminars (astro)

Adi Zolotov
Ohio State University

TBA

December 12, 2013 Thursday 3:30 PM 
Meyer 6th Floor Conference Room
Other Physics Department Events (other)

Physics Department Holiday Party

Light refreshments will be served.
RSVP to Lorelei.DeMesa@nyu.edu

April 10, 2014 Thursday 4:00 PM 
Meyer 122
Physics Colloquia (colloquia)

M Shaposhnokov

TBA